Referring to FIG. 1, the radio-frequency spectrum utilized in speed-determining radar devices is divided into a series of bands, with each band covering a range of frequencies within the radio-frequency spectrum. The frequencies of interest range from about 10.525 to 35.200 GHz., although all the frequencies within this range are not allocated for speed determining radars. The bands which are allocated for this purpose include: the X-band, which ranges from 10.500-10.550 GHz.; the K-band, which ranges from 24.050-24.250 GHz.; and the Ka band, which presently ranges from 34.200-35.200 GHz.
Radar detectors, used to detect the presence of radar speed detecting devices, must be able to sweep the entire radio-frequency spectrum of interest in a short period of time and yet must also be able to respond accurately to the presence of radio-frequency signals within the bands of interest. A radar detector includes an antenna which receives radiated radio-frequency electromagnetic waves and converts them into conducted radio-frequency electrical signals. These radio-frequency electrical signals are passed to a filter stage which prevents all signals, except those whose frequencies which are of interest, from passing through the filter stage to the detector stage. The detector stage provides the envelope detection useful for carrier detection in a swept receiver.
Since the radio-frequency spectrum of the speed determining radar devices is broad and since the filter stage and the detector stage are most easily constructed for, and most stable over, a narrow range of frequencies, the receiver just described is generally not used. Instead, the typical radar receiver heterodynes, or multiplies the received radio-frequency signal by a second signal, termed a local oscillator or LO signal, of a predetermined frequency, to produce an intermediate frequency or IF signal having a much lower frequency than the radio-frequency or rf signal. This IF-signal is then detected to produce an audio-frequency signal.
In this manner, by varying or sweeping the frequency of the local oscillator signal multiplying the received rf signal, the frequency of the intermediate frequency signal produced can remain constant over a wide range of rf-frequencies. Therefore, the filter stage and the detector stage can be optimized for a narrow band of intermediate frequencies.
In such a heterodyne or superheterodyne receiver, in addition to the above mentioned components, there is also included a sweeping local oscillator and a multiplier. The sweeping local oscillator is typically tunable over a broad frequency range and produces the predetermined local oscillator or LO frequency signal. The multiplier combines or heterodynes this predetermined local oscillator frequency signal with the radio-frequency signal to produce two intermediate frequency signals whose frequencies are the sum and difference of the radio-frequency signal frequency and the local oscillator signal frequency.
Typically, it is the difference intermediate frequency that is desired and the difference frequency is separated from the sum frequency by passing the output signals from the multiplier through a filter to select a filtered intermediate frequency signal. The filtered intermediate frequency signal is then passed to the detector for demodulation.
The radar detector herein disclosed is capable of sweeping all the rf-bands allocated to speed determining radar while reducing the amount of time required to perform the sweep.